Metal carbamates formed from diaminophenylmethane

ABSTRACT

The invention provides metal carbamates of the general formula (I) 
                         
where R 1  and R 2  are each an alkyl group.

REFERENCE TO PRIOR APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/918,931, filed Aug. 23, 2010; which is a 371 of PCT/EP09/53168, filedMar. 18, 2009, and priority to European patent application 08152934.9,filed Mar. 18, 2008, is claimed, of which the entire content isincorporated herein by reference.

DESCRIPTION

The invention provides metal carbamates formed from diaminophenylmethane(MDA) and a process for preparing them.

Carbamates and the preparation and use thereof are known.

For the preparation of carbamates and urethanes, a series of processesis known.

In these processes, for example, Lewis acids, for example uranium salts(U.S. Pat. No. 3,763,217), aluminum turnings with iodine and Hgpromoters (U.S. Pat. No. 4,550,188), zinc salts, iron salts, antimonysalts and tin salts (U.S. Pat. Nos. 4,268,683, 4,268,684, EP 391473),are used as catalysts. A disadvantage for the industrial use of theseprocesses are the sometimes low conversions, low selectivities or both.

High selectivities and yields are obtained, for example, in processescatalyzed with Lewis acids (Pb salts as catalysts), when a high excessof dialkyl carbonate (amine:carbonate 1:20) is used (WO 98/55451, WO98/56758). The high excess of dialkyl carbonate leads to large recyclestreams.

In other cases, high yields of urethane can be achieved when the ureaformed in the urethanization is redissociated thermally to thecorresponding urethane in an additional reaction (EP 048371 (catalysts:lead salts, titanium salts, zinc salts and zirconium salts), EP 391473(catalyst: Zn salts)). The redissociation requires an additional,energy-intensive step.

A further disadvantage in the case of use of Lewis acids as homogeneouscatalysts is the catalyst residues which remain in the product and canbe removed only incompletely.

WO 2007/015852 describes the use of Lewis acidic heterogeneous catalystsfor the urethanization of aromatic amines. This dispenses with acomplicated removal of a homogeneous catalyst. The resulting conversionsare too low for industrial scale applications and decrease together withthe selectivity with increasing lifetime of the heterogeneous catalyst.

It is also known that urethanes can be prepared from aromatic aminesusing basic compounds, for example, alkali metal or alkaline earth metalalkoxides.

DE 3202690 describes the preparation of aromatic urethanes by reactionof aniline and dialkyl carbonates in the presence of a small amount of ametal alkoxide as a catalyst. The conversions described in the examplesare incomplete and the selectivities achieved are insufficient for anindustrial application.

Journal of Organic Chemistry, 2005, 70, 2219-2224 describes the reactionof aniline with a large excess of dimethyl carbonate (40-fold excess) inthe presence of an excess of base such as sodium methoxide (NaOMe) orpotassium tert-butoxide (KOtBu). With NaOMe, a selectivity of 67% aftera reaction time of 210 min was obtained. With KOtBu, a selectivity after1 min of 100% is described, which, however, declines to 60% throughformation of the N-methylcarbanilate by-product with increasing reactiontime. Conversions and isolated yields were not described.

N-arylcarbamates can be converted to isocyanates. Such processes arecommon knowledge. This procedure allows diisocyanates to be prepared bya phosgene-free route. Such processes are used to prepare aliphaticdiisocyanates in particular.

In the case of aromatic diisocyanates, preparation by a phosgene-freeprocess is difficult, since a series of side reactions proceed owing tothe high reactivity of the aromatic compounds. However, it would bedesirable if aromatic diisocyanates, which are industrially of greatsignificance, were also preparable by phosgene-free processes.

It was an object of the present invention to find a simple means ofproviding starting materials for the preparation of aromaticdiisocyanates by a phosgene-free process, which can be prepared with ahigh selectivity, a high yield and with high purity.

It has been found that, surprisingly, it is possible to isolate metalcarbamates based on diaminophenylmethane (MDA) in pure form. Afterreaction with protic compounds, especially with alcohols or preferablywith water, these can be converted to the corresponding diurethane (MDU)and, in a subsequent step, by thermal cleavage to MDI (methylenediphenyl diisocyanate).

The invention accordingly provides metal carbamates of the generalformula (I)

where R₁ and R₂ are the same or different and are each an alkyl grouphaving 1-18 carbon atoms and M is an alkali metal atom.

Particular preference is given to alkyl groups having 2-7 carbon atoms,which may be branched, unbranched or cyclic, especially branched orunbranched.

In a preferred embodiment of the invention the R₁ and R₂ groups areidentical.

The invention further provides a process for preparing metal carbamatesof the general formula (I) by reacting

-   -   a) diamino diphenylmethane with    -   b) an alkyl carbonate of the general formula (II)

where R₁ and R₂ are each as defined above and

-   -   c) a metal compound of the general formula (III)    -   M(R₃)n        where    -   M is an alkali metal atom,    -   R₃ are the same as OR₁ and OR₂, or are an amide or an        alkylsilazide and    -   n is equal to 1.

In one embodiment of the invention the alkyl chain R₁ and/or R₂ ismodified with heteroatoms. The heteroatoms may be halogen atoms,preferably fluorine atoms and/or chlorine atoms, more preferablyfluorine atoms. In another embodiment, the heteroatoms are oxygen atoms.These are preferably present in the form of ether groups.

It has been found that the urethanes which have been prepared usingdiaklyl carbonates having heteroatoms in the alkyl group can be cleavedparticularly readily to form isocyanates.

R₁ and/or R₂ is preferably an ethyl, propyl, butyl, 2-methylpropyl,3-methylbutyl, n-pentyl, 2-methoxyethyl, 2-ethoxyethyl or a2,2,2-trifluoroethyl group.

R₁ and R₂ are more preferably identical. This has the advantage that, inthe course of preparation of the inventive products (I) and in thecourse of any further processing to urethanes and conversion thereof toisocyanates, fewer products are in the process.

The compounds of the general formula (I) are solid at room temperatureand can be removed from the reaction solution without any problem and inhigh purity. If required, they can be purified in a further processstep.

The compounds of the general formula (I) are prepared, as describedabove, by reaction of components a), b) and c).

The diaminophenylmethane (MDA) used may be any isomers in any mixingratios. Preference is given to using 2,4′-diaminophenylmethane,4,4′-diaminophenylmethane, 2,2′-diaminophenylmethane and higher homologs(polyaminopolyphenylmethanes) and isomer mixtures.

In a preferred embodiment of the invention, the dialkyl carbonates b)are selected from the group comprising diethyl carbonate, di-n-propylcarbonate, di-n-butyl carbonate, di-2-methylpropyl carbonate,di-3-methylbutyl carbonate, di-n-pentyl carbonate, bis-2-methoxyethylcarbonate, bis-2-ethoxyethyl carbonate, bis-2,2,2-trifluoroethylcarbonate.

The metal compound c) preferably comprises basic organic metalcompounds, especially compounds of alkali metals. They may, for example,be compounds comprising nitrogen atoms, for example amides, such assodium amide or compounds comprising silicon atoms and nitrogen atoms,for example lithium hexamethyldisilazide.

The base more preferably comprises the alkoxides of alkali metals. Thealkali metal M is preferably lithium, sodium or potassium. The alcoholradicals preferably corresponds to those of the alkyl carbonates of thegeneral formula (II) used.

The compounds of the general formula (I) are prepared preferably understandard pressure at temperatures between 100 and 150° C. The yield ofthe process is between 95-100%.

In the reaction, the ratio of carbonate groups to amino groups is from1:1 to 10:1, more preferably from 2:1 to 3:1.

The metal compound c) is preferably used in a stoichiometric amount,more preferably in a molar ratio of 1:1, based on the amino groups, i.e.in a ratio of about one mole of base per amino group.

The inventive metal carbamates may, as described, be converted to pureMDU by reprotonation with water.

The fact that a simple process would be able to prepare pure metalcarbamates and the object of the invention would thus be achieved wasnot foreseeable to the person skilled in the art.

It was also unnecessary to work with a high excess of component b). Inspite of the different reactivity of the two amino groups of the MDA,there was homogeneous conversion of the two amino groups.

The invention claimed is:
 1. A process for preparingmethylenediphenyldiurethane, the process comprising: reacting a compoundof water with a metal carbamate of formula (I):

wherein: R₁ and R₂ are each independently an alkyl group comprising 1 to18 carbon atoms; and M is an alkali metal ion.
 2. The process of claim1, wherein, in formula (I), the alkyl groups R₁ and R₂ each comprise2-18 carbon atoms in the chain.
 3. The process of claim 1, wherein, informula (I), the alkyl groups R₁ and R₂ each comprise 2-7 carbon atomsin the chain.
 4. The process of claim 1, wherein, in formula (I), thealkyl groups R₁ and R₂ are each independently an ethyl, propyl, butyl,2-methylpropyl, 3-methylbutyl, n-pentyl, 2-methoxyethyl, 2-ethoxyethyl,or a 2,2,2-trifluoroethyl group.
 5. The process of claim 1, wherein, informula (I), the alkyl groups R₁ and R₂ comprise heteroatoms.
 6. Theprocess of claim 5, wherein the heteroatoms are halogen atoms.
 7. Theprocess of claim 5, wherein the heteroatoms are fluorine atoms.
 8. Theprocess of claim 5, wherein the heteroatoms are chlorine atoms.
 9. Theprocess of claim 5, wherein the heteroatoms are oxygen atoms.
 10. Theprocess of claim 5, wherein the heteroatoms are oxygen atoms present inthe form of ether groups.
 11. The process of claim 1, wherein, informula (I), the alkali metal ion is lithium.
 12. The process of claim1, wherein, in formula (I), the alkali metal ion is sodium.
 13. Theprocess of claim 1, wherein, in formula (I), the alkali metal ion ispotassium.
 14. The process of claim 1, wherein, in formula (I), whereinR₁ and R₂ are identical.
 15. The process of claim 2, wherein, in formula(I), R₁ and R₂ are identical.
 16. The process of claim 3, wherein, informula (I), R₁ and R₂ are identical.
 17. The process of claim 4,wherein, in formula (I), R₁ and R₂ are identical.
 18. The process ofclaim 5, wherein, in formula (I), R₁ and R₂ are identical.